emissions of black carbon, organic, and inorganic aerosols...

Emissions of black carbon, organic, and inorganic aerosolsfrom biomass burning in North America and Asia in 2008

Y. Kondo,1 H. Matsui,1 N. Moteki,1 L. Sahu,1,2 N. Takegawa,1 M. Kajino,1,3 Y. Zhao,4

M. J. Cubison,5 J. L. Jimenez,5 S. Vay,6 G. S. Diskin,6 B. Anderson,6 A. Wisthaler,7

T. Mikoviny,7 H. E. Fuelberg,8 D. R. Blake,9 G. Huey,10 A. J. Weinheimer,11

D. J. Knapp,11 and W. H. Brune12

Received 7 October 2010; revised 12 January 2011; accepted 28 January 2011; published 19 April 2011.

[1] Reliable assessment of the impact of aerosols emitted from boreal forest fires on theArctic climate necessitates improved understanding of emissions and the microphysicalproperties of carbonaceous (black carbon (BC) and organic aerosols (OA)) and inorganicaerosols. The size distributions of BC were measured by an SP2 based on the laser‐inducedincandescence technique on board the DC‐8 aircraft during the NASA ARCTAS campaign.Aircraft sampling was made in fresh plumes strongly impacted by wildfires in NorthAmerica (Canada and California) in summer 2008 and in those transported from Asia(Siberia in Russia and Kazakhstan) in spring 2008. We extracted biomass burning plumesusing particle and tracer (CO, CH3CN, and CH2Cl2) data. OA constituted the dominantfraction of aerosols mass in the submicron range. The large majority of the emitted particlesdid not contain BC. We related the combustion phase of the fire as represented by themodified combustion efficiency (MCE) to the emission ratios between BC and otherspecies. In particular, we derived the average emission ratios of BC/CO = 2.3 ± 2.2 and8.5 ± 5.4 ng m−3/ppbv for BB in North America and Asia, respectively. The difference inthe BC/CO emission ratios is likely due to the difference in MCE. The count mediandiameters and geometric standard deviations of the lognormal size distribution of BC in theBB plumes were 136–141 nm and 1.32–1.36, respectively, and depended little on MCE.These BC particles were thickly coated, with shell/core ratios of 1.3–1.6. These parameterscan be used directly for improving model estimates of the impact of BB in the Arctic.

Citation: Kondo, Y., et al. (2011), Emissions of black carbon, organic, and inorganic aerosols from biomass burning in NorthAmerica and Asia in 2008, J. Geophys. Res., 116, D08204, doi:10.1029/2010JD015152.

1. Introduction

[2] Boreal forest fires are one of the most importantsources of aerosols transported to the Arctic [e.g., Stohl et al.,2006, 2007; Treffeisen et al., 2007; Eck et al., 2009]. Largeamounts of aerosols, predominantly in the form of carbona-ceous aerosols, namely black carbon (BC) and organicaerosols (OA), are emitted from biomass burning (BB). Theseparticles strongly absorb and scatter solar visible radiation(downwelling and upwelling), influencing the radiationbudget in the Arctic. They can also act as cloud condensationnuclei (CCN), which influence the microphysical propertiesof clouds [Lubin and Vogelmann, 2006]. Deposition of BConto snow and ice changes the surface albedo [Warren andWiscombe, 1980; Clarke and Noone, 1985], contributing towarming of the Arctic [Hansen and Nazarenko, 2004;Flanner et al., 2007, 2009]. Statistically, Russia and Canadahave been the largest sources of BC emitted from boreal forestfires [Lavoué et al., 2000; Stocks et al., 2002; Conard et al.,2002; Soja et al., 2004]. Fire activity may increase in thecourse of significant warming occurring prominently at highlatitudes [Stocks et al., 1998].

1Research Center for Advanced Science and Technology, University ofTokyo, Tokyo, Japan.

2Now at Physical Research Laboratory, Ahmedabad, India.3Now at Meteorological Research Institute, Tsukuba, Japan.4Air Quality Research Center, University of California, Davis,

California, USA.5Department of Chemistry and Biochemistry and CIRES, University of

Colorado at Boulder, Boulder, Colorado, USA.6NASA Langley Research Center, Hampton, Virginia, USA.7Institute of Ion Physics and Applied Physics, University of Innsbruck,

Innsbruck, Austria.8Department of Meteorology, Florida State University, Tallahassee,

Florida, USA.9Department of Chemistry, University of California, Irvine, California,

USA.10School of Earth and Atmospheric Sciences, Georgia Institute of

Technology, Atlanta, Georgia, USA.11Atmospheric Chemistry Division, National Center for Atmospheric

Research, Boulder, Colorado, USA.12Department of Meteorology, Pennsylvania State University,

University Park, Pennsylvania, USA.

Copyright 2011 by the American Geophysical Union.0148‐0227/11/2010JD015152

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, D08204, doi:10.1029/2010JD015152, 2011

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http://dx.doi.org/10.1029/2010JD015152

[3] Reliable estimates of the effects of BC on the climateof the Arctic by global models necessitate quantitativeunderstanding of emissions of BC, aging and removal, andmicrophysical properties [e.g., Stier et al., 2006; Textoret al., 2006; Koch et al., 2009]. More specifically, calcu-lations of the effects of light absorption by BC requirephoto‐absorption cross sections and size distributions ofBC. The cross section depends on diameter, and mixingstate, together with the refractive index and shapes of BC[e.g.,Moteki et al., 2010;Moteki and Kondo, 2010; Shiraiwaet al., 2010]. The size distributions of BC are controlledby emissions, transport, and deposition of BC. The coatingof BC by nonrefractory species also influences the CCNactivity of BC [e.g., Riemer et al., 2004; Oshima et al.,2009; Kuwata et al., 2009] and thus the efficiency ofwet deposition, partly compensating for the amplification oflight absorption [Stier et al., 2006]. The effect of BC onglobal and regional climate is currently poorly representedby models [e.g., Textor et al., 2006; Koch et al., 2009],owing in large part to the lack of constraints on emissionsand the evolution of these parameters.[4] Previous estimates of emission factors of BC are

considered to be highly uncertain, because they are based onmeasurements with large uncertainties [Reid et al., 2005]. Inparticular, interference by coemitted organic aerosol (OA)can cause large errors in BCmeasurements by thermal opticaltransmittance and filter‐based absorption photometers [e.g.,Reid et al., 2005; Kondo et al., 2009, 2011]. Reliablemeasurements of BC in the Arctic in the spring of 2008using a single particle soot photometer (SP2) were reportedby Warneke et al. [2009, 2010]. They observed transport ofenhanced BC emitted from BB in Siberia and Kazakhstanover the Alaskan Arctic together with BC‐CO correlations.[5] There is a critical need to characterize the emission

ratios and microphysical properties of BC near BB sourcesbefore being transported over long distances. We also needto understand the evolution of the microphysical propertiesof BC after emission for improved representation of theseparameters and processes by models. The major objectivesof this study are to elucidate these key parameters andprocesses of BC using a well‐calibrated SP2 based on laser‐induced incandescence (LII) [e.g., Moteki and Kondo, 2007,2008; Schwarz et al., 2006, 2008; Kondo et al., 2011].[6] For this purpose, we made measurements of BC and

light‐scattering particles (LSP) in the submicron range bythe SP2 on board the NASA DC‐8 aircraft over Alaska andCanada during the Arctic Research of the Composition ofthe Troposphere from Aircraft and Satellite (ARCTAS) mis-sion in the spring (ARCTAS‐A) and summer (ARCTAS‐B)of 2008 [Jacob et al., 2010]. Frequent aircraft samplingswere made in plumes from boreal forest fires that occurredin Canada and California in summer and in Asia (Siberia andKazakhstan) in spring. It is noted that in 2008, the borealwildfires in southern Siberia and agricultural fires inKazakhstan both occurred unusually early in spring andwere much more intense than usual at that time of year[Warneke et al., 2009].[7] In Canada, aerosol measurements in large plumes

were made in the vicinity of the burning areas 1–2 h afterthe emissions. We have derived the emission ratios of aero-sols from BB based on the measurements with unprecedentedaccuracy. We sampled emissions from wildfires emitted

under varying combustion conditions, namely flaming andsmoldering fire phases. We systematically interpret thedependence of the emission ratios on the conditions ofcombustion.[8] The air masses impacted by BB in Asia were sampled

2–3 days after the emissions. We identified the locations offires that influenced concentrations of BC by using kine-matic back trajectories. We selected data little influenced bywet deposition during transport to estimate the emissionratios. These observations have enabled detailed compar-isons of the aerosol emission ratios and their microphysicalproperties in the two regions.[9] The most important parameters and processes pre-

sented by this study are the evolution of the size distributionsand mixing state (coating thickness) of BC, the emissionratios of BC versus CO and CO2, and their dependence on theconditions of combustion. We have also elucidated the dif-ference of these parameters between the BB plumes fromAsia and North America, which are the two major regions ofboreal forest fires. These parameters can be used directly forimproving model estimates of the impact of BB on climate,especially in the Arctic.

2. Aircraft Observations and Methodologyof Data Analysis

2.1. Instrumentation

[10] The data used for this study are the mass, volume,and number concentrations of BC and LSP, mixing ratios ofcarbon dioxide (CO2), carbon monoxide (CO), acetonitrile(CH3CN), dichloromethane (CH2Cl2), sulfur dioxide (SO2),dimethyl sulfide (DMS), and reactive nitrogen (NOx, NOy,PANs, and HNO3), and submicron aerosol chemical com-position. The instruments used for the present study andtheir measurement accuracies are summarized in Table 1,together with their references.[11] The number concentrations of BC and LSP and the

mixing state of BC (coating thickness of BC by non-refractory components) were measured by the SP2. Themeasurement principle and schematic diagram of the SP2have been described previously [Gao et al., 2007; Motekiand Kondo, 2007], and detailed descriptions of the cali-bration of the SP2 used for ARCTAS are given elsewhere[Moteki and Kondo, 2010; Kondo et al., 2011]. We sum-marize the important points below.[12] Each BC particle sampled by the SP2 was irradiated

by a laser beam at a wavelength of 1064 nm and thus heatedto incandescence. The SP2 monitors LII signals in twodistinct visible bands (l = 300–550 nm and 580–710 nm).In addition, laser light scattered by individual particles wasdetected by two avalanche photodiodes to measure the sizesand number concentrations of LSP.[13] BC particles can be identified by the ratio of LII

intensities of the two visible bands, because the ratio is aproxy for the vaporization temperature of the particles. Thevaporization temperature of ambient BC was measured to beabout 4000 K [Schwarz et al., 2006]. The BC mass for eachparticle (mSP2) was estimated from the LII peak intensity.We measured the LII peak intensities of monodisperse BCaerosols that were mass selected by an aerosol particle massanalyzer (APM; Model‐302, KANOMAX, Inc., Japan) witha 400°C heated inlet [Moteki and Kondo, 2007]. BC in

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ambient air was extracted by vaporization of the condensedcompounds coating the BC particles by means of the heatedinlet. The SP2 calibration using ambient BC was made forseveral hours on 10 November 2009. The error in extractingrefractory BC mass is estimated to be less than about 10%[Kondo et al., 2011]. This extraction procedure means thatthe definition of the mass of ambient BC is the massof incandescent aerosols refractory at 400°C (mref*). Theuncertainty of mref* is about 10%.[14] The LII peak intensity increased linearly with mass

up to about 20 femtograms (fg). At higher mass, the re-lationships deviated from linearity. We used the LII peakintensity– mref* relationship for ambient BC to derive theBC size distributions. In this way, mSP2 is fully traceable tothe calibration standards (mref*).[15] The mass equivalent diameters of the BC cores (DBC)

were calculated by assuming a density (rBC) of 2 g cm−3,which is also close to the value of 1.77 g cm−3 derived byPark et al. [2004]. About 10% uncertainty in rBC led toerrors of a few % in DBC. The DBC detection range of theSP2 used for this study was 80–860 nm. The absoluteaccuracy of the measurements of mass concentration of BCis estimated to be about 10% [Kondo et al., 2011].[16] The diameters of the coated BC particles (shell

diameters, Dp) were derived using the scattering signals for

Dp. The coating thickness of BC or shell/core ratio (Dp/DBC

ratio) is a useful conceptual parameter to represent themixing state of BC. A detailed description of the method toderive Dp is given by Kondo et al. [2011].[17] LSP scatters laser light but does not incandesce. The

peak scattering signal was converted to the scattering crosssection of the particles. Calibration was made first by usingpolystyrene latex (PSL) particles with a refractive index of1.59 + 0i. The calibration curve of PSL converts to that of(NH4)2SO4 with a refractive index of 1.52 + 0i based on theMie theory algorithm developed by Bohren and Huffman[1998]. The diameters of LSP (Dp) were estimated fromthis curve with an uncertainty of about 10% in the size rangeof about 200–750 nm [Moteki and Kondo, 2008]. We used(NH4)2SO4 in defining Dp because it generally constitutesan important fraction of PM1 and its refractive index iswell known. Refractive indices of a number of OA werereported to range between 1.50 and 1.63 by Jacobson [1999].Refractive indices of OA emitted by BB were measured to bein the range of about 1.56–1.64 at visible wavelengths [Levinet al., 2010]. Refractive indices of SOA photochemicallygenerated from a‐ and b‐pinene and toluene were measuredto range from 1.4 to 1.5 at l = 670 nm [Kim et al., 2010].[18] Figure 1 shows the calculated scattering cross sections

of particles with refractive indices of 1.5 ± 0.1 at a wave-length of 1064 nm based on Mie theory. The changes inparticle diameters for changes in the refractive index of ±0.1from 1.5 + 0i for the same scattering cross sections in thediameter range of 200–500 nm is within 10%, as summa-rized in Table 2. The uncertainty in Dp by possible changesin the refractive indices of LSP is estimated to be about 10%

Table 1. Measurements During ARCTAS

Measurement Technique Accuracy References

BC SP2 ±10% Moteki and Kondo [2007, 2010]LSP number distribution SP2 ±10% Moteki and Kondo [2007, 2010]LSP volume distributiona SP2 ±30%Aerosol chemistry AMS ±35% DeCarlo et al. [2006, 2008]

and Dunlea et al. [2009]CO TDLAS ±2% Sachse et al. [1987]CO2 NDIR gas analyzer ±0.25 ppmv Vay et al. [2003]CH3CN PTR‐MS ±10% Wisthaler et al. [2002]CH2Cl2 WAS‐GC – Blake et al. [2003]DMS WAS‐GC – Blake et al. [2003]SO2 (CIT) CIMS ±(50% of measurement value + 100 pptv) Crounse et al. [2006, 2009]SO2 (GIT) CIMS – Slusher et al. [2004] and Kim et al. [2007]NOx and NOy Chemiluminescence ±10% and ±15% Weinheimer et al. [1994]PANs CIMS Slusher et al. [2004] and Kim et al. [2007]HNO3 CIMS Crounse et al. [2006, 2009]OH LIF ±40% Brune et al. [1999]

aEstimated by uncertainty propagation from the number distribution.

Figure 1. Calculated scattering cross sections as a functionof particle diameter for different refractive indices at a wave-length of 1064 nm.

Table 2. Comparison of Particle Diameter Dp (N) in Units of nmEstimated for Different Refractive Indices (N) and Their Ratios

Dp(1.5) Dp(1.4) Dp(1.6) Dp(1.4)/Dp(1.5) Dp(1.6)/Dp(1.5)

200 214 189 1.070 0.945250 269 236 1.076 0.944300 324 283 1.080 0.943350 380 328 1.086 0.937400 437 374 1.093 0.935450 494 418 1.098 0.929500 551 463 1.102 0.926


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considering the reported refractive indices of inorganic andorganic aerosols. The total mass concentrations of BC (MBC)and volume concentrations of LSP (VSC) were derived byintegrating the size distributions with respect to diameter.The propagated uncertainty of VSC from the uncertainty ofDp is estimated to be about 30%. We did not convert VSC tothe mass concentration of LSP because the densities of LSPare not provided by the SP2 measurements. The values ofMBC and VSC are given in the units of mass and volume perunit volume of air at standard temperature and pressure(STP; 273.15 K and 1013 hPa), respectively.[19] Sample air was provided to the instruments using a

forward facing, window‐mounted inlet probe. Real‐timemeasurements of air speed, static pressure, and temperaturewere used to maintain isokinetic flow through the inlet tipto minimize inertial enhancement or depletion of coarseparticles. Detailed measurements made during the DC‐8 Inlet Characterization Experiment (DICE) in the spring of2003 showed that the inlet efficiently transmits both dustand sea salt particles smaller than 4 mm in dry diameter[McNaughton et al., 2007].

2.2. Air Sampling of Fire Plumes

[20] Figure 2 shows the flight tracks of the NASADC‐8 aircraft, flown from Cold Lake (54°N, 110°W),Alberta, Canada, between 29 June and 10 July 2008, and thetransit flight to Palmdale, CA on 13 July. On 29 June, 102fires were burning and more than half of those ignitionsresulted from lightning strikes that occurred within the pre-vious 24 h (A. J. Soja et al., ARCTAS: The perfect smoke,in The Canadian Smoke Newsletter, pp. 2–7, Fall 2008).Most of the plumes were encountered over SaskatchewanProvince, while some of them were measured over theYukon Territory and Manitoba. A substantial portion of theflight time was devoted to sampling forest fire plumes withina few hours after emission mostly in the boundary layer(BL) and the lower free troposphere (FT). Figure 2 showsthe locations of the sampling of the BB plumes used for thepresent study that includes data obtained in forest fireplumes in California during the transit flight.

[21] Large‐scale BB occurred in Siberia (boreal forestfires) in Russia and in Kazakhstan (agricultural fires) inMarch and April 2008 [Warneke et al., 2009, 2010]. Theair masses impacted by these fires were transported fromthese regions and were sampled by the local flights fromFairbanks, Alaska, in April 2008 during ARCTAS‐A. Theflight tracks of the DC‐8 during ARCTAS‐A are shown byJacob et al. [2010]. These data are used for comparison withthe ARCTAS‐B data.[22] Satellite‐derived column amounts of CO from the

Measurements of Pollution in the Troposphere (MOPITT)instrument [Drummond and Mand, 1996; Bowman, 2006]and aerosol optical depth (AOD) from the Moderate reso-lution Imaging Spectroradiometer (MODIS) [Remer et al.,2005] during ARCTAS‐A were similar to monthly averagesfor individual months over the Alaskan Arctic (the regionincluding both measurements and transport pathways ofmeasured air parcels) [Matsui et al., 2011]. This suggests thatARCTAS‐A data are representative of the North AmericanArctic in April 2008.[23] Air masses impacted by forest fires that occurred

in California were also sampled by the DC‐8 ARCTASinstrument payload on 18–24 June 2008 for the CaliforniaAir Resources Board (CARB) [Singh et al., 2010]. We usedrelevant data obtained during ARCTAS‐CARB for com-parison with the ARCTAS‐B data.

2.3. Backward Trajectories

[24] We used 10 day backward trajectories calculatedevery minute along the flight tracks using a kinematic model[Fuelberg et al., 2010, and references therein]. The meteo-rological data were from runs of the Weather Research andForecasting (WRF) model with a horizontal resolution of45 km and 50 vertical layers [Skamarock et al., 2008].Source regions were identified as the regions (projected ontothe surface) where the backward trajectories encounteredand remained in altitudes below 700 hPa (about 3 km abovesea level), which is the approximate altitude of the upperboundary of the BL over the land during daytime.[25] Air masses with origins outside Canada were excluded

for the analysis of the ARCTAS‐B data, which focuses onnear‐field emissions. Some of the air masses sampled duringARCTAS‐A were estimated to have originated from twodifferent biomass burning regions in Asia from 10 day backtrajectories.[26] We calculated the accumulated precipitation along

each trajectory (APT) usingGlobal Precipitation ClimatologyProject (GPCP) global precipitation data [Huffman et al.,2001; Adler et al., 2003]. We used daily data (version 1.1)with a horizontal resolution of 1 × 1 degree. The GPCPdata were derived solely from observations based on gaugemeasurements and estimates of rainfall by geostationary andpolar orbiting satellites. Precipitation at the surface (i.e.,vertically integrated) was summed up along the trajectoriesfrom the sampling location to the identified source regions.We used the APT data as a measure of wet removal ofwater‐soluble species during transport. The uncertainties inthe estimates of source regions and APT were calculated byusing two meteorological data sets for trajectory calcula-tions and three precipitation data sets, including the GPCPdata. The comparison has shown that the general featuresof the statistics and conclusions obtained in this study do

Figure 2. Flight track of the DC‐8 over Canada duringARCTAS‐B. Phases of BB of the air masses sampled areshown with circles.


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not change with the choice of trajectory model andmeteorological and precipitation data sets. More detaileddescriptions of the uncertainties in the estimation of thesource regions and APT are given elsewhere [Matsui et al.,2011].

2.4. Extraction of Fire Plumes

[27] We first determined species background concentra-tions to quantify the effect of BB on the concentrations ofthe species measured on board the aircraft. They weredetermined as the 5th percentiles of the values in every 10 Krange of potential temperature, and they were linearlyinterpolated vertically. The background values of most ofthe species were close to zero, except for CO2 and CO. Thedifferences between the measured and background con-centrations (D values) were used for the analysis. The 1 minaveraged data with DCO > 20 parts per billion by volume(ppbv) were used for the statistical analysis to obtainemission ratios from individual data points with sufficientprecision maintaining a sufficient number of the data pointsfor statistical analysis.[28] We selected data strongly influenced by BB using the

concentrations of trace hydrocarbon species, CH3CN andCH2Cl2, as described in detail by Matsui et al. [2011].CH3CN is known to be emitted from forest fires [de Gouwet al., 2003]. CH2Cl2 is emitted from industrial solvents, butnot emitted from combustion of fossil fuels [Chen et al.,2007; Choi et al., 2003]. However, it can be used as atracer of anthropogenic emissions of aerosols because theemissions of solvent (CH2Cl2) generally coexist with emis-sions of species by fossil fuel combustion. The BB‐impactedair masses transported from Asia were defined as those withDCH3CN > 50 parts per trillion by volume (pptv) andDCH2Cl2 < 5 − 10 pptv. The threshold ofDCH2Cl2 increasedfrom 5 pptv atDCH3CN = 50 pptv to 10 pptv atDCH3CN >100 pptv, as depicted in Figure 3 ofMatsui et al. [2011]. Forthe Canadian BB, the data selection was made using thethresholds ofDCH3CN > 100 pptv andDCH2Cl2 < 10 pptv.

2.5. Modified Combustion Efficiency

[29] The phase of BB has been shown to be representedby the combustion efficiency (CE) or modified combustionefficiency (MCE) [e.g., Delmas et al., 1995; Yokelson et al.,1999, 2008]. The calculation of CE requires measurement ofincompletely oxidized compounds (CO) and reduced com-pounds such as methane, nonmethane hydrocarbons, andammonia, among others [Ward et al., 1991], while MCE isestimated only from the DCO/DCO2 ratio.

MCE ¼ DCO2=DCO2 þDCOð Þ ¼ 1= 1þDCO=DCO2ð Þ: ð1Þ

MCE is useful in interpreting observed emission ratios as afunction of MCE, irrespective of the relative amount ofdata in different combustion phases [Yokelson et al., 1999;Goode et al., 2000]. A MCE of 0.9 is sometimes treated asthe value classifying air masses predominantly influencedby combustion in the flaming phase (MCE > 0.9) whereas,MCE < 0.9 represents the smoldering phase [Reid et al.,2005]. The 1 s time resolution of CO and CO2 data led tothe correspondingly high time resolution of the MCE values.

This parameter was used for the interpretation of the dataimpacted by BB.

3. Aerosols in BB Plumes in Canadaand California During ARCTAS‐B

[30] In Figure 3 (top), we show the time periods of sam-pling BB plumes identified by the methods discussed insections 2.3 and 2.4. Figures 3 (top) and 3 (middle) show thetime series plots of CO, MBC, VSC, MCE, and NOx/NOy

ratio measured during Flight 18 (F#18) on 1 July. The APTand altitude of the DC‐8 are shown in Figure 3 (bottom).The MBC and VSC values were greatly enhanced during en-counters of wildfire plumes. In Figures 3 (top) and 3 (bottom),we show the time periods of sampling BB plumes identifiedby the methods discussed in sections 2.3 and 2.4. Generally,the MCE was lower than 0.95 in the relatively fresh plumes(NOx/NOy > 0.4) sampled during this flight.[31] Figure 4 shows the DMBC/DCO correlation versus

the estimated APT that the air masses experienced within24 h prior to sampling. The integration time in calculatingthe APT was fixed at 24 h, independent of the age of the BBplumes. Most of the Canadian and Californian air massesexperienced little precipitation (APT < 3 mm). For the anal-ysis of the ARCTAS‐B and CARB data, we selected datawith APT < 1 mm to minimize the effect of wet deposition onthe analysis of aerosols.[32] The first panel in Figure 5 (left) shows the DCO‐D

CO2 correlations used to derive the MCE for individualplumes. Depending on the DCO/DCO2 ratios, the phases ofcombustion of these plumes were classified as smoldering,flaming, and others. It should be noted that the D valueswere used for the statistical analysis throughout this study,and the linear regressions are used only for supplementaryanalysis. The locations of the measurements of the plumeswith the different combustion phases are shown in Figure 2.Smoldering phase plumes were measured mainly on 1 Julyduring F#18 (Figure 3), and flaming phase plumes weremeasured on 4 July (F#19) and 13 July (F#24 in California).The flaming phase data obtained in California are shownwith crosses. No smoldering phase data were obtainedin California during ARCTAS‐B. The correlations of theflaming phase data measured in Canada and Californiashowed no statistical differences, as discussed below. Con-sidering this, the data obtained in these regions weregrouped as North American for statistical analysis. Figure 6shows the profiles of MBC and VSC measured over NorthAmerica during ARCTAS‐B. The data points are colorcoded by the phase of BB. The color‐coded data are alsowith APT < 1 mm.[33] Figure 7 shows the average number and mass (or

volume) size distributions of BC and LSP observed in layersimpacted by wildfires over North America. A small dis-continuity in the BC mass size distributions near 200 nm is anumerical artifact in constructing the size distributions fromthe data in each size bin. The count median diameters(CMD) of the lognormal fitted BC size distributions wereunvarying at 136 ± 6 nm, with a geometric standard devi-ation (sgc) of 1.32, although the CMD slightly increasedwith the increase in MCE, as discussed in section 8. The sizedistributions of the LSP were also unvarying, and their


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average CMD and sgc are summarized in Table 3. TheMMD of BC and volume median diameters (VMD) of LSPwere 187 ± 10 nm and 294 ± 42 nm, respectively. Table 3summarizes these values, together with the geometric stan-dard deviations (sgm or sgv) of these size distributions. Theaverage fractions of the unmeasured MBC and VSC values(DBC < 80 nm and Dscat < 185 nm) were estimated to be 4%and 18%, respectively.[34] We also derived the median shell/core ratios (Dp/DBC)

of BC forDBC > 200 nm every 1min. Themedian value of theDp/DBC was 1.48 (1.42 for smoldering phase and 1.62 forflaming phase (Table 3)). Possible causes of the dependenceof Dp/DBC on MCE are discussed in section 8. The estimatedvalue using the above median of Vp/VBC (=Dp

3/DBC3 ) is 3.24

(2.86 for smoldering phase and 4.25 for flaming phase).Thus BC represents about 0.31 (=1/3.24) of the volume(ranging from 0.23 to 0.35 for flaming and smoldering,respectively) of the particles that contain BC. We mademore detailed analysis of the dependence of the micro-physical parameters of BC on MCE in section 8.

Figure 3. (top) Time series plots of CO, BC mass concentration (MBC), volume concentration of LSP(VSC), and identification of BB plumes. (middle) MCE and NOx/NOy ratio. (bottom) Precipitation(APT) and altitude of the DC‐8 on 1–2 July.

Figure 4. MBC/DCO ratios plotted versus APT duringARCTAS‐B. The APT was calculated for 24 h prior to air-craft sampling.


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[35] Figure 8 and Tables 4 and 5 show the average massconcentrations and mass fractions of submicron non-refractory chemical components and BC in the extracted BBplumes as measured by the High‐Resolution Time‐of‐Flight

AerosolMass Spectrometer (HR‐ToF‐AMS) [DeCarlo et al.,2006; Dunlea et al., 2009] and SP2 on board the DC‐8. Themass concentrations are in the units of mg STP m−3. Theaverage VSC/MBC and VSC/VBC ratios in the North American

Figure 5. Correlations between MBC, VSC, DCO2, DCO, and CH3CN in wildfire plumes over Canada(circles) and California (crosses) during ARCTAS‐B with APT < 1 mm. The data points are color codedfor smoldering phase (MCE < 0.9; blue), flaming phase (MCE > 0.95: red), and others (0.90 < MCE <0.95; green).


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plumes are summarized in Table 6. The uncertainties aregiven as the errors in the determinations of the slopes of thecorrelations shown in Figures 5 and 11. The prescribedmeasurement errors are not included in this uncertaintyestimate.[36] On average, OA constituted about 83% of the total

mass concentration of BC and nonrefractory aerosol with aDva (vacuum aerodynamic diameter [see DeCarlo et al.,2004]) smaller than about 1 mm (PM1). We therefore inferthat VSC in the plumes approximately represents the volume

concentration of OA. MBC was converted to BC volumeconcentration (VBC) assuming rBC = 2 g cm−3. The averageVSC/MBC and VSC/VBC ratios in the PM1 range are sum-marized in Table 6. The average volume fraction of BC wasabout 0.02. Together with the result above that the volumefraction of BC was 0.31 for particles containing BC, thisindicates that the large majority of the scattering particlesmeasured by SP2 in the size range of 200 nm < Dp < 750 nmdid not contain BC with DBC > 80 nm, which is consistentwith the particle number observations with the SP2.

Figure 6. Vertical profiles of (left) MBC and (right) VSC for all data points measured over Canada duringthe ARCTAS‐B campaign. The mass concentration of aerosols with density of 1 g cm−3 is 1 mg m−3 for avolume concentration of 1 mm3 cm−3.

Figure 7. Mean distributions for (top) BC in number and mass space and (bottom) LSP in number andvolume space, averaged for eight major forest fire plumes encountered in Canada during ARCTAS‐B andthose in BB fire plumes transported from Asia. Solid fitted lines are lognormal distribution functions.


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[37] Figure 5 shows the correlations between DCO2,DCO, DCH3CN, DMBC, and DVSC. The correlations(DCO‐DCO2,DCH3CN‐DCO,DMBC‐DCO2,DVSC‐DCO2,DMBC‐DCO, DVSC‐DCO, and DVSC‐DMBC) are colorcoded by the phase of BB. The least squares fits weremade for each phase and were constrained through the ori-gin. The flaming phase data obtained in Canada (red circles)and California (red crosses) showed no significant differ-ences. The variability in the slopes was smallest for theDCH3CN‐DCO correlation. The correlation depended littleon the phase of BB.

4. Aerosols in Plumes Transported From Asia(Siberia and Kazakhstan)

[38] Figure 9 shows time series plots of CO, MBC, VSC,and altitude of the DC‐8, measured on 12–13 April (F#8).The MBC and VSC values were enhanced during the en-counters with wildfire plumes. The time periods of samplingplumes of different origins in Asia are also shown in Figure 9.The plumes of the different origins appear as a number ofinterleaved spikes.[39] We extracted data from air masses strongly influ-

enced by biomass burning in Asia (Siberia and Kazakhstan).Most of the air masses enhanced in DMBC and DVSC weretransported from Asia in 4.5 ± 2.1 days according to thecalculated 10 day back trajectories.[40] Figure 10 shows the DMBC/DCO correlation versus

the estimated APT that the air masses experienced. Gener-ally BB plumes transported from Asia in spring experiencedsmall amounts of APT. The observed DMBC/DCO correla-

tion tends to decrease with APT, especially for APT > 20mm.We used the data with APT < 10 mm for the analysis of theemission ratios to minimize the possible effect of wetremoval during transport. The slopes changed little by se-lecting data with APT < 5 mm, although the number of datapoints was greatly reduced.[41] Figure 8 and Tables 4 and 5 show the average mass

concentrations and mass fractions of PM1 nonrefractorycomponents of aerosol and BC in the Asian BB plumesmeasured by HR‐ToF‐AMS and SP2. OA constituted thedominant fraction (about 60%) of the nonrefractory com-ponents, similar to the Canadian BB plumes. However, thesulfate mass fraction was as large as about 30%, which ismuch larger than that for the North American plumes (3%–8%). It is very likely that SO2 emitted from biomass burningwas oxidized to form sulfate during transport. Formation ofsulfate is discussed in more detail in section 7.[42] Table 6 shows the average VSC/MBC and VSC/VBC

ratios in the Asian plumes, restricted to low APT. Theseratios were about 2 times lower than those in the Canadianplumes. The conclusion that most of the light‐scatteringparticles did not contain BC is still valid. The difference inthe BC fractions is discussed in terms of the differences inthe conditions of combustion in section 6.[43] The total mass concentrations of nonrefractory com-

ponents measured by the AMS (MAMS) were highly corre-lated with VSC, with r

2 = 0.89 and 0.84 for ARCTAS‐A and B,respectively. The average densities of nonrefractory compo-nents derived from the least squares fits of theMAMS‐VSC were1.33 and 1.54 g cm−3 for ARCTAS‐A and B, respectively.[44] Aerosols strongly influenced by BB have been shown

to act as cloud condensation nuclei (CCN) [e.g.,Hallett et al.,1989; Vestin et al., 2007; Fors et al., 2010] due to the watersolubility of some organic compounds. Therefore, the BCparticles (thickly coated) in the Canadian and Asian plumeswill act as CCN. It is likely that BC particles from Asiawere also coated by sulfate, leading to higher CCN activitybecause of the high hygroscopicity of sulfate [e.g., Pettersand Kreidenweis, 2007]. However, actual BC removal byprecipitation depends on the timing of precipitation withrespect to the formation of sulfate during transport. It istherefore difficult to compare the efficiency of wet removalBC particles in Canadian and Asian plumes.[45] Figure 11 shows the correlations between DCO2,

DCO, CH3CN, DMBC, and DVSC of the plumes (with

Table 3. Size Distributions of BC and LSP

Asia Canada

BCCMD 141 ± 8 nm 136 ± 6 nmsgc 1.37 ± 0.04 1.32 ± 0.01MMD 207 ± 31 nm 187 ± 10 nmsgm 1.54 ± 0.1 1.48 ± 0.04Dp/DBC 1.52 1.48

LSPCMD 238 ± 11 nm 224 ± 14 nmsgc 1.31 ± 0.03 1.33 ± 0.05VMD 306 ± 40 nm 294 ± 42 nmsgv 1.46 ± 0.1 1.58 ± 0.08

Figure 8. Fractions of major chemical components of aerosols measured by AMS in major forest fireplumes in (left) Canada and (right) Asia.


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APT < 10 mm) transported from Siberia and Kazakhstanduring the ARCTAS‐A period. We defined mixed airmasses as those in which the trajectories passed over bothregions. The plumes from the different source regions inAsia were classified and are color coded in Figure 11. Thecorrelations of air masses transported from Kazakhstan andSiberia did not show significant differences. The resultswere the same even if we used the data set without discrim-ination by APT. Because of the small differences betweenSiberian and Kazakhstan air masses, we grouped these andmixed air masses as the Asian air masses for the statisticalanalysis discussed hereafter.[46] Warneke et al. [2009] have shown significant differ-

ences in the slopes of correlations between various atmo-spheric species (e.g., CH3CN‐CO, DMBC‐DCO, DCO‐DCO2,OA (DVSC)‐DCO) for plumes originating from Siberia and

Kazakhstan by using the Lagrangian particle dispersionmodel FLEXPART [Stohl et al. 2005]. The reasons for thedifference between theirs and the present results are notunderstood, although it may be partly due to the differencein identifying the source regions by using different methodsor to variability within each source region coupled with thesampling of different plumes by the two aircraft.[47] It should be noted that the DCO/DCO2 ratios can

change during the course of dilution of the BB plumesthrough mixing with ambient air [e.g., Takegawa et al.,2003, 2004]. The effect of dilution can be larger for lon-ger‐lived species with higher background values, such asCO2. Initial changes in the DCO/DCO2 ratios of the BBplumes occur when they are injected into the FT. However,generallyDCO2 andDCO in the BB plumes are much largerthan the difference in the background CO2 and CO mixing

Table 5. Averages and Standard Deviations of the Ratios of Mass Concentrations of BC, Inorganic, and Organic Aerosols to PM1 in wt %for the Smoldering (MCE < 0.90) and Flaming Phases (MCE > 0.95) During ARCTAS and CARB

Species Asia, All

ARCTAS‐B CARB

AllMCE < 0.90(1 July)

MCE > 0.95(4 and 13 July) All MCE < 0.90 MCE > 0.95

BC 4.9 ± 1.1 2.7 ± 2.9 1.7 ± 0.6 3.4 ± 0.6 2.5 ± 1.0 1.9 ± 0.2 2.8 ± 1.2Cl− 0.3 ± 0.2 0.2 ± 0.2 0.2 ± 0.1 0.1 ± 0.1 0.4 ± 0.2 0.6 ± 0.2 0.4 ± 0.1NH4

+ 6.1 ± 1.7 3.6 ± 2.6 1.7 ± 0.6 4.8 ± 2.5 3.8 ± 1.5 5.5 ± 0.8 3.8 ± 1.5NO3

− 2.6 ± 1.6 1.8 ± 1.7 1.9 ± 0.7 1.7 ± 1.5 1.4 ± 1.0 1.2 ± 0.2 1.6 ± 1.0SO4

2− 29.6 ± 9.1 7.7 ± 7.1 2.8 ± 0.9 9.8 ± 7.1 8.0 ± 4.8 13.0 ± 2.3 8.0 ± 4.6SO4

2− + SO2 31.1 ± 9.6 11.9 ± 1.3 4.0 ± 1.3 20.0 ± 14.8 15.6 ± 9.4 19.6 ± 3.5 14.8 ± 8.5Org 56.3 ± 8.6 84.2 ± 11.4 91.7 ± 1.8 80.3 ± 9.6 83.9 ± 6.5 77.8 ± 3.0 83.5 ± 6.5

Table 4. Concentrations of Species for BB Plumes During ARCTAS and CARB

Species Units

Asia BB,ARCTAS‐A,All, Flaming

NA BB, ARCTAS‐B Canada BB,ARCTAS‐B,MCE > 0.95,

Flaming (4 July)

CA BB, CARB

All

MCE < 0.90,Smoldering(1 July)

MCE > 0.95,Flaming

(4 and 13 July) AllMCE < 0.90,Smoldering

MCE > 0.95,Flaming

Number 89 353 125 96 13 105 26 50DCO ppbv 40.8 124 328 111 54.7 125 413 90.6DCO2 ppmv 2.79 2.17 1.33 4.36 5.57 2.51 1.89 2.78DCH3CN pptv 85.3 194 536 170 141 211 570 163BC ng m−3 273 331 517 348 348 298 1240 280LSP mm3 cm−3 3.51 8.78 13.6 8.74 6.21 8.57 22.7 5.56Shell/core ratio – 1.52 1.48 1.42 1.62 1.63 1.51 1.56 1.46Cl− mg m−3 0.0192 0.0253 0.0653 0.0135 0.0258 0.0472 0.0848 0.0351NH4

+ mg m−3 0.398 0.674 0.812 0.534 0.564 0.383 0.743 0.325NO3

− mg m−3 0.138 0.259 0.847 0.13 0.427 0.226 0.553 0.151SO4

2− mg m−3 1.67 1.2 1.19 0.921 0.951 0.607 1.09 0.49SOx mg m−3 2.02 1.78 1.75 2.42 1.06 1.85 4.49 1.52Org mg m−3 3.69 12.2 40.5 9.11 8.7 9.55 46.5 8.46PM1 sum mg m−3 6.19 14.69 43.93 11.06 11.02 11.11 50.21 9.74PM1 + Av SO2 6.54 15.27 44.49 12.56 11.13 12.36 53.61 10.77NO pptv 15.4 51.3 46.6 64.9 17.3 60.7 39.7 64.1NO2 pptv 16.5 153 171 221 41 287 371 236NOy pptv 881 1158 1598 1582 845 1970 3067 1637HNO3 pptv 15.6 110 43.6 449 179 463 516 463PAN pptv 498 405 712 349 422 400 922 237NOx/NOy – 0.0362 0.176 0.136 0.181 0.069 0.176 0.134 0.183SO2(CIT) pptv 91.9 144 131 532 NA 294 794 246SO2 pptv 73.6 126 129 170 24.6 292 794 235Average SO2 pptv 83 135 130 351 24.6 293 794 241SO2/DCO pptv/ppbv 2.0 1.1 0.39 3.2 0.45 2.3 1.9 2.7SOx/DCO pptv/ppbv 11 3.2 1.2 5 4.3 3.3 2.5 3.9SO2 as SO4

2− mg m−3 0.35 0.58 0.56 1.5 0.11 1.25 3.4 1.03DMS pptv NA 6 8 2.83 NA 7.5 20 6


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ratios between the BL and FT. Therefore, the changes in theslopes of the CO‐CO2 correlations by this process shouldbe small.[48] Larger changes in the DCO/DCO2 ratios are con-

sidered to occur by mixing of the diluted BB plumes withambient air during the long‐range transport in the FT. Theaverage CO2 mixing ratios at the surface and in the FT wereobserved to increase by about 1 ppmv between 50°N and70°N [e.g., Komhyr et al., 1985; Tans et al., 1989; Andersonet al., 1996]. In addition, the mean CO2 mixing ratios of the1 km averaged values between 2 and 7 km were 391.7 ±0.7 ppmv during ARCTAS‐A. If 50% of the BB plumesare mixed with ambient air during transport from 50°N to70°N, the CO2 mixing ratio of the mixed air is anticipated

to increase by 0.5 ppmv on average. This leads to anuncertainty of theDCO/DCO2 ratios of about 15%–20% forthe average DCO2 of 2.8 ppmv (Table 4), giving a measureof the uncertainties in the derived DCO/DCO2 for the AsianBB plumes.[49] The size distributions of BC and LSP in the plumes

from Asia (APT < 10 mm) are shown in Figure 7. The CMDand MMD (VMD) of the lognormal fitted BC and LSP sizedistributions together with the values of the standard de-viations are summarized in Table 3. The values for theplumes from Siberia and Kazakhstan were very similar.Therefore they were grouped together and labeled as Asian.The variability of these parameters was small. The medianDp/DBC ratios for the Asian plumes was 1.52 (corresponding

Figure 9. Time series plots of CO, BCmass concentration (MBC), and volume concentration of LSP (VSC)measured over Alaska on 12–13 April. Bars indicate the periods when the plumes from the respectiveregion were sampled.

Table 6. Emission Ratios of BC, CO, CO2, and CH3CN for Biomass Burninga

Species

Asia BB,ARCTAS‐A,All, Flaming

NA BB, ARCTAS‐BCA BB,CARB,All, Mix

Kazakhstan,b

ARCPAC

LakeBaikal,b

ARCPAC

Andreaeand Merlet[2001],Mix

CA FF,c

CARB

TokyoFF,d

2009All, MixMCE < 0.90,Smoldering

MCE > 0.95,Flaming

CO/CO2 (ppbv/ppmv) 15 ± 5 80 ± 110 222 ± 118 26 ± 10 114 ± 23 50 ± 25 42 ± 19 107 ± 38 5.2 ± 0.9 11MBC/CO (ng m−3/ppbv) 8.5 ± 5.4 2.3 ± 2.2 1.7 ± 0.8 3.4 ± 1.6 3.4 ± 1.4 10 ± 5 7 ± 4 6.5 ± 3.2 3.1 ± 1.0 5.7MBC/CO2 (ng m−3/ppmv) 129 ± 67 180 ± 269 369 ± 257 88 ± 80 236 ± 103 NA NA 701 ± 245 16.2 ± 8.2 62MBC/CH3CN (ng m−3/pptv) 3.9 ± 2.1 1.2 ± 1.4 0.88 ± 0.6 2.2 ± 1.3 1.7 ± 0.6 NA NA NA Nil NilCH3CN/CO (pptv/ppbv) 2.2 ± 1.6 1.8 ± 0.4 1.9 ± 0.4 1.6 ± 0.74 1.7 ± 0.4 NA NA NA Nil NilVSC/MBC (cm3 m−3/g m−3) 12 ± 3 24 ± 6 26 ± 7 24 ± 6 23 ± 6 NA NA NA 5 ± 3 NAVSC/VBC (cm3/cm3) 24 ± 6 48 ± 12 52 ± 14 48 ± 12 46 ± 12 NA NA NA 10 ± 6 NAVSC/CO (mm3 cm−3/ppbv) 10 ± 2.3 5.4 ± 1.6 4.4 ± 2.3 8.3 ± 4.3 – NA NA NA NA NAVSC/CO2 (mm

3 cm−3/ppmv) 1.6 ± 0.7 4.4 ± 4.0 9.6 ± 8.0 2.1 ± 0.9 – NA NA NA NA NA

aThe mass concentrations are in units of mg STP m−3.bWarneke et al. [2009].cFossil fuel combustion in California.dFossil fuel combustion in Tokyo [Kondo et al., 2010].


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to a BC volume fraction of 0.28 for BC‐containing parti-cles), indicating that the BC particles were thickly coated.The size distributions of BC and LSP and the Dp/DBC

ratios of the Asian fire plumes were very similar to thoseof the North American fire plumes (Figure 7), despite thesignificant differences in the emission ratios, including theDCO/DCO2 ratios.

5. Average Emission Ratios

[50] We derived the emission ratios between MBC, VSC,CH3CN, CO, and CO2 averaged over the measurementperiod during ARCTAS‐B. The averaging method describedby Andreae and Merlet [2001] was used for this purpose.Namely, the net increases of species X and Y above theirbackground values in the jth plume were first calculated(DXj and DYj). Then the sums for all the plumes werecalculated, i.e., DX = ∑j DXj and DY = ∑j DYj. The averageemission ratio of X versus Y was given as ERav = DX/DY.We note that for VSC the value derived is not strictly an“emission” ratio, as secondary inorganic (e.g., sulfate, asdiscussed above) and organic aerosol species [e.g., DeCarloet al., 2010] are formed during transport and solar exposure.[51] We derived the emission ratios of CO/CO2, MBC/CO,

MBC/CO2, MBC/CH3CN, CH3CN/CO, VSC/CO2, VSC/CO,and VSC/MBC, and they are summarized in Table 6. The largevariability in MBC/CO, MBC/CO2, and MBC/CH3CN arereadily seen. The lower variabilities in the CH3CN/CO emis-sion ratio is also seen.[52] The emission ratios for BB in Asia, discussed in

section 4, were derived in the same way and are given inTable 6. In Table 6, we added the reported values for bio-mass burning in Asia for April 2008 during the ARCPACMission [Warneke et al., 2009]. In addition, we also includedthe data for forest fires in California during ARCTAS‐CARBin Table 6 for comparison.[53] The average CO/CO2 (orMCE) andMBC/CO emission

ratios were similar for wildfires during ARCTAS‐B (Canadaand California) with DCO/DCO2 = 80 ± 100 ppbv/ppmv

and ARCTAS‐CARB (California) withDCO/DCO2 = 114 ±23 ppbv/ppmv. In the Asian plumes, the average CO/CO2

emission ratios were much smaller with DCO/DCO2 = 15 ±5 ppbv/ppmv, and the MBC/CO emission ratios were a fewtimes larger, although the MBC/CO2 emission ratios weresimilar.[54] Estimates of the emission ratios for biomass burning

in previous studies were compiled by Andreae and Merlet[2001]. The compiled values for temperate forests are alsolisted in Table 6. The present MBC/CO and MBC/CO2

emission ratios for wild fires in North America are lowerthan the compiled value by a factor of 2–4, although theCO/CO2 emission ratio was close to their estimates. Thederived MBC/CO emission ratio for BB plumes from Asiawas much closer to the compiled value, although theobserved MBC/CO2 emission ratio was about 5 times lower.[55] Table 6 also shows the VSC/MBC ratios observed in

plumes strongly influenced by emissions by combustion offossil fuel in California (ARCTAS‐CARB) and in Tokyo inAugust–September 2009 [Kondo et al., 2011]. These datawere taken from the same SP2 calibrated in the same way,enabling direct comparison. The comparison shows thatmuch larger volumes of LSP relative to those of BC werecoemitted with BC from the wildfires than those from fossilfuel combustion. Generally the emission ratios of primaryorganic aerosols to BC have been reported to be of the orderof 1 when scaled with the mass concentrations [Takegawaet al., 2006] for fossil fuel combustion, consistent with theVSC/VBC ratio for Tokyo in Table 6. The variability in theVSC/MBC ratios in Tokyo was caused mainly by the for-mation of secondary aerosols (organic and inorganic) [e.g.,Kondo et al., 2011], consistent with results at many otherlocations [de Gouw and Jimenez, 2009].[56] OA/BC ratios were observed for aerosol particles

emitted from crown fires in Canadian wildfires in a previousstudy [Conny and Slater, 2002]. These authors report thatBC/TC (OC + BC) ratios were 0.085 and 0.0087 for theflaming and smoldering stages, respectively. The corre-sponding OC/BC ratios were about 10 and 100 for thesestages, consistent with the present results. OA/BC and OC/BCmass ratios of about 6 and 4 were observed in agriculturalfire plumes in the Yucatan [Yokelson et al., 2009]. Theseratios are much lower than the present results and thosereported by Conny and Slater [2002].

6. Dependence of the Emission Ratios on MCE

6.1. Injection Height

[57] The injection heights of the plumes formed in dif-ferent phases of burning can vary because of the differencein the thermal energy release as a function of the burningphase. This point was suggested by Andreae and Merlet[2001] but has not been fully demonstrated by observedatmospheric species. To investigate this possible effect, thesampling altitudes of the plumes are plotted versus theDCO/DCO2 ratios and MCE in Figure 12. It is seen thatthe DCO/DCO2 ratios decreased and MCE increased withaltitude, indicating a strong dependence of these parameterson the phase of burning.[58] The thermal energy received by the plumes during

burning increases their potential temperature (PT) from anaverage surface value of �0 to �fire. The plumes with �fire (>�0)

Figure 10. MBC/DCO ratios plotted versus APT duringARCTAS‐A.


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Figure 11. Correlations between MBC, VSC, DCO2, DCO, and CH3CN observed in major wildfireplumes that experienced little precipitation during transport from Asia to Alaska (ARCTAS‐A). The leastsquares linear regression fits are shown by solid lines.


13 of 25

gain buoyancy and ascend to the altitude where the PT of theambient air (�a) is equal to �fire. Cloud formation will notsubstantially alter �fire, if clouds evaporate without precipi-tation. Therefore D� = �a − �0 is considered to be propor-tional to the thermal energy gained per unit mass of theplume near the surface. We estimated D� by assuming that�a is the PT of the sampled plume and �0 is the average PTnear the surface during each flight, for plumes sampled nearthe point of emission. Active mixing in the BL duringdaytime by solar heating of the surface will smooth out thevertical structure of D� produced by the thermal energy ofcombustion. Therefore the derived D� should be interpretedas a representation of the injection height that accounts forthe vertical thermodynamic structure near the BB sources inNorth America.[59] Figure 12 (bottom) shows the relationships of the

DCO/DCO2 ratio and MCE with D� for the North Amer-ican plumes. The sharp increase in D� at MCE > 0.98 isclearly seen. Namely, a substantial increase in D� wasdetected only at the highest combustion efficiencies.[60] For comparison, the data for the Asian plumes are

also plotted in Figure 12. We estimated �0 for these plumesby combining the calculated trajectories and the surface PTof the burning areas in Asia estimated using NCEP data. Theuncertainty of �0 was estimated from the variability in thetemperature of the source region. Because we cannot narrowdown the source region from the trajectory analysis, theuncertainty in �0 was as large as about 10 K. However,because the sampled plumes were injected directly into theFT, the uncertainty in estimatingD� caused by mixing in theBL should be smaller than for the North American plumesmeasured in the BL. Despite this uncertainty, the increase in

D� corresponded with an MCE as high as 0.98, similarly tothe North American plumes.[61] It is likely that plumes formed during the flaming

phase were selectively injected to higher altitudes than thoseduring the smoldering phase over Asia. The higher‐altitudeplumes were transported in shorter time to the sampling re-gions near Alaska, associated with higher wind speeds[Fuelberg et al., 2010]. In other words, the DCO/DCO2

ratios of the Asian plumes measured during ARCTAS‐Amaynot represent the vertically averaged emission values in Asia.The estimation of relative contributions of the flaming andsmoldering phases in producing aerosols and trace gasesincluding CO will require data in the BL over Asia.

6.2. Emission Ratios

[62] We have attempted to interpret the large variabilityof the slopes of the DMBC‐DCO, DMBC‐DCO2, DMBC‐DCH3CN, DVSC‐DCO2, DVSC‐DCO, and DVSC‐DMBC

correlations in terms of MCE. These ratios are plotted versusMCE in Figure 13. Standard deviations (1s) of the 1 mindata used to derive individual data points were calculatedfirst. Table 7 summarizes the averages of the 1s values andthe average ratios for BB plumes in Asia, Canada, andCalifornia shown in Figure 13. The least squares fits werederived using the data sets with APT < 1 mm for Canadaand California and APT < 5 mm for Asia. It should be notedthat some of the emission ratios (DVSC/DCO2,DMBC/DCO2,DMBC/DCO, DVSC/DCO) are not independent of MCE,because the estimate of MCE contains DCO2 and DCO.[63] The DMBC/DCO ratio increased with the increase in

MCE. This relation is important because the ratio is oftenused in estimating BC emissions and BC removal during

Figure 12. (top) Relationship between the sampling altitudes of the plumes versus DCO/DCO2 ratiosand MCE. Vertical bars represent the variability of the estimate. The least squares linear regression fitsare also shown. (bottom) Relationships between the change in potential temperature (with reference tothe surface level potential temperature) versus DCO/DCO2 ratio and MCE.


14 of 25

transport using CO as a tracer. Because of the large variabilityin theDMBC/DCO ratios, the estimates of BC removal basedon downwind measurements of DMBC/DCO will have largeuncertainty unless DMBC/DCO was measured at the source.In contrast, the DMBC/DCO2 and the DVSC/DCO2 ratiosdecreased with the increase in MCE. TheDVSC/DMBC ratiosalso decreased with the increase in MCE.[64] For flaming fires, the lower DVSC/DMBC ratios tend

to lower the single scattering albedo (SSA) of the ensembleof aerosols. The thicker coating of BC for the flaming phaseleads to amplified absorption as compared with that forexternally mixed BC. The SSA of individual BC particlesincreases with the increase in the shell/core ratios because ofthe increase in the scattering cross section [Oshima et al.,2009]. However, the amplified BC absorption leads to adecrease in the SSA of the ensemble of aerosols [e.g., Shiraiwaet al., 2008; Seinfeld and Pandis, 2006, Figure 21.11]. For thisreason, we expect a lower (higher) SSA in plumes of theflaming (smoldering) phase. In fact, the SSA at a wavelengthof 538 nm was observed to decrease substantially with theincrease in MCE in BB plumes in Brazil [Reid and Hobbs,1998].

[65] TheDMBC/VSC (DVSC/DMBC) ratio showed an almostexponential increase at MCE > 0.95. The rapid increase inBC/OC at MCE > 0.95 was observed by combustion facilityexperiments [Christian et al., 2003], consistent with thepresent results, considering that OA is the dominant com-ponent of DVSC.[66] The DVSC/DCO ratios increased with the increase in

MCE, although the correlation was not tight. The interpre-

Figure 13. Scatterplots of MBC/CO, MBC/CO2, MBC/CH3CN, CH3CN/CO, VSC/CO2, VSC/CO, and VSC/MBC ratios versus MCE. Linear regression fits using all the data points are also shown.

Table 7. Averages of the 1s Values (±) and the Average Ratios,MCE, and Shell/Core Ratios for BB Plumes in Asia, Canada, andCalifornia

Asia Canada California

DVSC/DCO2 1.88 ± 0.40 7.97 ± 2.30 2.50 ± 1.16DVSC/DMBC 0.0116 ± 0.0011 0.0261 ± 0.0053 0.0246 ± 0.0019DMBC/DCO 11.8 ± 4.5 3.25 ± 0.678 2.86 ± 0.35DVSC/DCO 0.133 ± 0.051 0.0749 ± 0.0116 0.0699 ± 0.0053DMBC/DCO2 167 ± 37 357 ± 117 100 ± 48DMBC/DCH3CN 5.16 ± 1.44 2.10 ± 0.56 2.12 ± 0.34MCE 0.985 ± 0.0023 0.846 ± 0.060 0.961 ± 0.021Shell/core ratio 1.54 ± 0.033 1.45 ± 0.026 1.51 ± 0.033


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tation of this relationship is not straightforward becauseDVSC and DCO are both smoldering products. This trendmay be sensitive to the relative emissions (and secondaryformation) of LSP and CO to those of CO2 because boththe DVSC/DCO2 and DCO/DCO2 ratios decreased with theincrease in MCE.[67] A number of studies have shown the particle emis-

sion factor (g/kg) to decrease with the increase in MCE byburning tropical forest fuels in a combustion facility [Fereket al., 1998; Yokelson et al., 2007, 2008]. The particle massand fuel mass are approximately proportional to DVSC andDCO2 +DCO. Their results are qualitatively consistent withthe present observations, although the types of fuels weredifferent, namely tropical versus boreal forests. The agree-ment in the trend between our study and those of Yokelsonet al. [2007, 2008] provides additional support for themethodology in using MCE in the interpretation of fielddata. The present analysis demonstrates the necessity oftaking into account the dependence of the emission ratios ofaerosols on the burning phase (e.g., MCE) in improving theuncertainty in estimating aerosol emissions from BB.

7. Formation of Sulfate

[68] Here we discuss sources of sulfate in BB plumes.First, it should be noted that air masses strongly impacted byanthropogenic activities were excluded using CH2Cl2 asa tracer throughout the present analysis, as discussed insection 2.4. This already minimizes the effects of SO2

emissions from anthropogenic activity near the BB regionson the Canadian and Asian plumes.[69] Second, we investigated the effect of mixing of BB

air masses with air masses with unidentified origins, namelyair masses that did not originate from identified regions ofBB or anthropogenic emissions in the BL within 10 days.Table 8 summarizes the average SO2 and sulfate concentra-tions in the background air and in the Asian BB plumes. Thesulfate concentrations in the background air were 3–4 timeslower than those in the Asian BB plumes, although the SO2

concentrations were similar. Considering this, it is alsounlikely that mixing of FT air caused elevated sulfate in theBB‐impacted air masses during the transport from Asia inthe FT.[70] Now we discuss sources of sulfate in BB plumes

based on the statistical data shown in Tables 4 and 5. InTables 4 and 5, mass concentrations of sulfate, which canform from SO2, are also shown. In the Canadian plumes,sulfate was only 3% of PM1 in the smoldering phase, and it

was higher (8%) in the flaming phase. However, the massfraction of the observed PM1 accounted for by sulfate pluspotential sulfate = (SO4

2− + f × SO2)/(PM1 + f × SO2) wasas high as 20% for MCE > 95% and it remained low forMCE < 90%. Here f = 4.28 mg m−3/ppbv is the conversionfactor from gas phase SO2 concentrations in ppbv to equiv-alent sulfate mass concentrations in mg m−3. The dependenceof the SOx = SO2 + SO4

2− on MCE was similar for the CARBdata. In the Asian plumes (flaming phase), SO4

2− and SOx

constituted about 27% and 31% of PM1, respectively.[71] The comparison of BB in different regions also in-

dicates that SOx was emitted from BB most intensively inthe flaming phase, as observed by Ferek et al. [1998]. Alarge fraction of the emitted SO2 was oxidized to sulfateduring transport from Asia to the Alaskan region (versus amuch smaller oxidized fraction for the Canadian plumes),leading to the higher fraction of sulfate in the Asian plumesthan those from the other regions. The longer time scale ofoxidation of sulfate versus secondary organic aerosol (SOA)precursors also results in changing OA/sulfate ratios foranthropogenic pollution plumes, which first increase asSOA forms and then decrease as sulfate forms on a longertime scale [Brock et al., 2008; Dunlea et al., 2009].[72] Emissions of SO2 from BB were also observed in the

Yucatan [Yokelson et al., 2009] and in laboratory experi-ments simulating wildfires [Yokelson et al., 1996; Burlinget al., 2010], while direct emission of sulfate has also beenobserved in similar laboratory experiments [Lewis et al.,2009]. The fires in the Yucatan were primarily crop resi-due fires, similar to those in Kazakhstan. The emission ratiosfrom these studies are somewhat higher than those estimatedby Andreae and Merlet [2001]. The molar SO2/CO emissionratios were about 2% in the Yucatan, and they were about1%–2% for evergreen coniferous fuels [Yokelson et al.,1996]. These SO2/CO emission ratios are about an orderof magnitude higher than the present observations. It ispossible that the variations in SO2 emissions between regionsvary with the amount of sulfate and SO2 deposited ontovegetation from the atmosphere and directly by farming,which will be larger for areas with large SO2 sources (such asMexico) and lower for areas without such large sources (suchas Saskatchewan).[73] DMS was also emitted by BB, as summarized in

Table 4, although at levels 1–2 orders of magnitude lowerthan for SO2. The DMS/SOx ratios for Canadian plumesindicate more efficient DMS emissions in the smolderingphase than in the flaming phase. For the CARB data, theratios were similar for smoldering and flaming phases. Theincreased emission of the reduced form of sulfur‐containingspecies in the smoldering phase is in contrast to the increasedemission of the oxidized form of sulfur (SO2) from theflaming phase.

8. Microphysical Properties of BC and LSP

8.1. Size Distribution

[74] The number and mass (volume) size distributions ofaerosols in the BB plumes from North America and Asiawere fitted to a good approximation with lognormal sizedistribution functions, with typical examples shown inFigure 7. To a first approximation, these parameters were verysimilar in all the BB plumes. For more detailed investigations,

Table 8. Comparison of SO2 and Sulfate Concentrations inBackground Air and BB Plumes From Asia During ARCTAS‐AWith and Without Selection by APT

Instrument All APT < 10 mm

SO2 (pptv)Background CIT 98 116

GIT 78 101Asian BB CIT 108 140

GIT 81 105

Sulfate (mg m−3)Background AMS 0.13 0.14Asian BB AMS 0.46 0.49


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Figure 14 compares the average CMD and standard deviation(sgc) of BC and LSP and the median Dp/DBC ratios in theNorth American and Asian plumes as a function of MCE.Table 7 summarizes the averages of the 1 s values of theDp/DBC ratios together with the average Dp/DBC ratios. Theleast squares fits were derived using the data sets withAPT < 1 mm for Canada and California and APT < 5 mm forAsia. Some of the plumes in Canada were measured after1–2 h after emissions, suggested from the NOx/NOy ratios(Table 4).[75] As a whole, the CMD and sgc of BC did not change

substantially without depending on the location of the fires,MCE, and age, although the CMD increased and sgc decreasedslightly with the increase in MCE. The rate of growth BC bycoagulation depends strongly on the initial BC concentration[e.g., Hinds, 1999; Seinfeld and Pandis, 2006]. After thisinitial growth, BC particles thickly coated by mainly organiccompounds probably grew much more slowly by coagula-tion under largely diluted conditions. Coagulation generallyincreases the CMD and decreases sgc [Kajino and Kondo,2011]. Figure 14 shows that the CMD and sgc of BC slightlyincreased and decreased, respectively, with the increase in

MCE. The observed anticorrelation of CMD and sgc of BC isqualitatively consistent with this effect.[76] The small variability of the size distributions of

LSP measured by SP2 is also noteworthy, considering thedifferences in the vegetation and therefore different chemi-cal compounds constituting the OA particles. The CMD andsgc increased slightly with the increase in MCE. Theseare the first extensive measurements of the size distributionsof BC and LSP (mainly OA) emitted from boreal forestfires.[77] The CMD and MMD of BC impacted by BB near the

Gulf of Mexico were about 140 nm and 200 nm [Schwarzet al., 2008], confirming the small variability of the sizedistributions of BC from BB. The ages of the BB plumeswere estimated to be about 0.5–1.5 h. These observationssimplify modeling efforts in expressing the size distributionsof primary particles emitted from boreal forest fires. Namely,the BC and LSP number size distributions with ages olderthan a few hours are represented to a good approximation bylognormal distribution functions, with an average CMD =138 nm and sg = 1.35 and an average CMD = 230 nm andsg = 1.32, respectively.

Figure 14. CMD and sg of (top) BC and (middle) LSP and (bottom) median shell/core ratios plottedversus MCE for the Asian and North American BB plumes. Linear regression fits using all the data pointsare also shown.


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8.2. Evolution of the Mixing State of BC

8.2.1. Observed Features[78] We investigated the evolution of the mixing state of

BC for different phases of combustion mainly using theCanadian data, which contain data with largely different airmass ages. We used NOx/NOy and NOy/DCO ratios as ameasure of aging with different time scales. Figure 15 showsthe median Dp/DBC (SC) ratio versus the NOx/NOy andNOy/CO ratios measured in the Canadian plumes. The datapoints are color coded with the phase of combustion. In thesmoldering phase, the median Dp/DBC ratio was ∼1.35for fresh plumes with NOx/NOy ∼ 0.8. The average OHconcentrations in the Canadian plumes were about 4 ± 2 ×106 cm−3. The ages of air masses with these NOx/NOy ratiosare estimated to be 1–2 h considering oxidation of NOx onlyby OH, neglecting the formation of PAN [Kondo et al., 2008;Alvarado et al., 2010]. This indicates that BC was thicklycoated by organics 1–2 h after emission.[79] For smoldering phase emissions, the Dp/DBC ratio

increased to 1.45 at NOx/NOy ratios of 0.05–0.1. The ages

of plumes are roughly estimated to be about 12–24 h. Theincrease in the Dp/DBC ratio from 1.35 to 1.45 correspondsto a 40 ± 30% increase of the volume of the coating mate-rials. It should be noted that the estimate in the increasedepends sensitively on the choice of the Dp/DBC ratio. Forthe plumes with MCE between 0.9 and 0.95 (green points inFigure 15), the increase in volume can be as high as 80%,although the scatter is substantial.[80] For the flaming phase plumes, we used the NOy/

DCO ratios as indicators of photochemical age, becausethese plumes were more aged than the smoldering plumes.The Dp/DBC ratios clearly increased with the decrease in theNOy/CO ratios shown in Figure 15 (bottom). The increase inthe Dp/DBC ratio from 1.4 at NOy/DCO = 30 to 1.66 atNOy/DCO = 10 corresponds to a 205 ± 40% increase ofthe volume of the coating materials.[81] In very fresh BB plumes in Australia, the NOy/DCO

ratios were observed to increase with the increase with MCE[Takegawa, 2001]. They were about 40 pptv/ppbv and20 pptv/ppbv at MCE = 0.96 and 0.92, respectively, similarto the present observations. NOx emitted is converted toPAN and HNO3, followed by conversion of PAN to HNO3

and dry deposition of HNO3 [e.g., Mauzerall et al., 1998;Takegawa et al., 2003; Alvarado et al., 2010]. About 60% ofNOy was observed to decrease within 2–3 days [Takegawaet al., 2003]. Most of the data shown in Figures 15 and 17were obtained in the BL. The main conclusions discussed inthis section are unchanged even if the BL data were selec-tively used.[82] The changes in the NOx/NOy, PAN/NOy, and HNO3/

NOy ratios in the plumes in the flaming phase are shown inFigure 16, in order to understand the processes of thechanges in the NOy. It can be seen that about 70% of NOx

was lost by conversion to PAN and HNO3 already at anNOy/DCO ratio of 30 pptv/ppbv. The decrease in the NOy/DCO ratios occurred mainly through the deposition ofHNO3, produced from NOx. It is estimated that the increasein the Dp/DBC ratios continued longer than about 2–3 days,considering the time scale of the NOy loss, through HNO3

deposition, as discussed above.

Figure 15. Median shell/core (SC) ratio plotted versus NOx/NOy andNOy/DCO ratios in BB plumes during ARCTAS‐B.The data points are color coded for the smoldering phase(MCE < 0.9; blue), flaming phase (MCE > 0.95: red), andothers (0.90 < MCE < 0.95; green).

Figure 16. NOx/NOy, PAN/NOy, and HNO3/NOy ratiosversus NOy/DCO ratios in the flaming phase plumes shownin Figure 15.


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[83] Figure 17 shows the DVSC/DMBC and OA/DMBC

ratios versus the NOy/DCO ratio. Figure 17 indicates nosubstantial increase in DVSC and OA relative to DMBC withage. Therefore it is unlikely that condensation of secondaryaerosol species formed from precursor gases emitted directlyfrom BB caused the increase in the Dp/DBC ratios. Since thefraction of VSC accounted for by the BC‐containing particlesis very small, a potential explanation for this apparent dis-crepancy is that OA was evaporating from particles thatcontained OA but not BC and then recondensing (possiblyafter gas phase oxidation) on the BC‐containing particles inthe SP2 size range [Marcolli et al., 2004; Robinson et al.,2007; Grieshop et al., 2009].8.2.2. Effect of Coagulation[84] Brownian coagulation may also explain the observed

evolution of the BC mixing state. Figure 18 shows the totalnumber concentration of LSP particles (Nsc) versus theNOy/DCO ratio. Generally Nsc was less than 3000 cm−3 and

1000 cm−3 in the BB plumes in smoldering and flamingphases, respectively. We use these data in calculating theeffect of coagulation, as detailed below.[85] A simple coagulation model was used to assess the

effect of coagulation between LSP and BC on the evolutionof the BC mixing state. The model solves for the intramodalcoagulation and intermodal coagulation between the twomodes, namely LSP (mode i) and BC (mode j). The evo-lution of number size distribution functions of LSP (ni (vpi))and BC (nj (vpj)) due to intramodal coagulation can be ex-pressed as [Whitby and McMurry, 1997],

d

dtn vp� � ¼ 1

2

Zvp

0

� ~vp; vp � ~vp� �

n ~vp� �

n vp � ~vp� �

d~vp

�Z∞

0

� vp;~vp� �

n vp� �

n ~vp� �

d~vp ð2Þ

where vp and n (vp) are the volume of a particle and numbersize distribution of mode i or j, respectively, and b is theBrownian coagulation kernel [Binkowski and Shankar, 1995].Here vp = pDp

3/6, assuming that all particles are spherical.The evolutions of ni (vpi) and nj (vpj) due to intermodalcoagulation are expressed as follows.

d

dtni vpi� � ¼ �

Z∞

0

� vpi; vpj� �

ni vpi� �

nj vpj� �

dvpj

d

dtnj vpj� � ¼

Zvpj

0

� vpi; vpj � vpi� �

ni vpi� �

nj vpj � vpi� �

dvpi

�Z∞

0

� vpj; vpi� �

nj vpj� �

ni vpi� �

dvpi ð3Þ

Figure 17. DVSC/DMBC and OA/DMBC ratios plotted ver-sus the NOy/DCO ratio in the flaming phase plumes shownin Figure 15.

Figure 18. Total number concentrations of LSP particlesplotted versus the NOy/DCO ratios during ARCTAS‐Bfor the smoldering phase (MCE < 0.9; blue), flaming phase(MCE > 0.95: red), and others (0.90 < MCE < 0.95; green).


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We assumed that the collision of a LSP particle (diameter ofDpi) with a BC particle (shell diameter of Dpj) forms aspherical BC particle with a shell diameter of (Dpi

3 + Dpj3 )1/3,

considering the conservation of volume concentration. Ini-tially, n (vp) was assumed have a lognormal size distribution(LNSD) function. The coagulation processes represented byequations (2) and (3) were calculated using the difference(bin) method, allowing deviations from the LNSD function.[86] As initial conditions, the LNSD parameters were

given covering the range of values shown in Figures 14 and18. Namely, NSC = 500, 1500, and 2500 cm−3, NBC = 0.1 ×NLSP, CMDSC = 200, 230, and 250 nm, CMDBC (shell) =120, 160, and 200 nm, sSC = 1.2 and 1.4, and sBC = 1.2 and1.3. The detailed model settings are the same as BIN100 ofKajino [2011], in which the size distribution was dividedinto 100 bins between 1 nm and 1 mm, and 100 bins between1 mm and 1 mm. The high accuracy of BIN100 is shown inhis study.[87] Figure 19 shows the total decay ofNSC due to intramodal

and intermodal coagulation, the decay due solely to in-tramodal coagulation (DNSC (intra)), and the decay due solelyto intermodal coagulation (DNSC (inter)) for NSC = 500 cm−3

(Figure 19a) andNSC = 2500 cm−3 (Figure 19b).DNSC (intra)andDNSC (inter) are shown as positive values. It can be seenthat the decrease in NSC is insensitive to the LNSD para-meters, such as CMD and s in the given range. In otherwords, the rates of coagulation are similar for the individualparticles defined by these parameters. The rate of coagula-tion is higher at larger NSC, as would be anticipated.[88] The number of LSP colliding with BC particles in 1–

2 days is nearly the same as the number of BC particles perunit volume of air. Namely, one BC particle collides withone LSP particle once in 1–2 days, on average. The collisionof one BC and one LSP particle with similar sizes resultsin a 1.26 times (∼21/3) increase in the SC ratio (Dp/DBC).The SC ratio is predicted to increase from 1.2 to 1.5 by

coagulation in 1–2 days. This result generally points out theimportance of coagulation with LSP in increasing SC ratioswith aging, in addition to condensation.[89] Now we translate the results of Figure 19 into the

changes of the volumes of shells coating BC cores. Figure 20shows time series of the ratio of the total shell volume of BCat time t (V (t) =

Rvp n(vp, t) dvp) to the initial V (t = 0) for some

selected cases. The initial LNSD parameters are shown inFigure 20. Figure 20a is the result of calculations for con-ditions close to those for the observations. CMDBC (shell) =160 nm corresponds to a SC ratio of about 1.33 for CMDBC

(core) = 120 nm. The lines with triangles, squares, and circlesare the results for initial CMDSC = 250, 230, and 200 nm,respectively. Figure 20b is the same as Figure 20a but forCMDBC (shell) = 120 nm (i.e., SC ratio = 1 forCMDBC (core) =120 nm). Figures 20c and 20d are the same as Figure 20a, butfor an initialNSC of 1500 and 2500 cm

−3. The increase in V (t)is faster for larger CMDSC in Figures 20a–20d.[90] As discussed above, the rates of coagulation are

insensitive to the parameters that define LNSD. Therefore, itis the difference in the size of LSP that contributes primarilyto the difference in the rate of increase in the SC ratio or V (t).Generally, larger sSC and larger differences between sSC andsBC result in faster increases in the SC ratio because of thismechanism. In fact, the rate of increase in SC is largest forsSC = 1.4 and sBC = 1.3 within the range of the parametersat the fixed CMDs. Therefore, the rates of changes in the SCratio at sSC = 1.2 and sBC = 1.3 shown in Figure 20 are thelowest values.[91] Figures 20a and 20b also show that the larger differ-

ence between CMDBC and CMDSC leads to the faster changesin the SC ratios for the same reason. The rate of the increase inthe SC ratio is larger for higher number concentrations, asshown by the comparison of Figures 20a, 20c, and 20d. TheV (t)/V (0) ratio increases by factors of 1.5–3 (factors of1.14–1.44 scaled for Dp/DBC) in 1–2 days for the conditions

Figure 19. Total decay of NLSP due to intramodal and intermodal coagulation, the decay due solely tointramodal coagulation (DNLSP (intra)), and the decay due solely to intermodal coagulation (DNLSP

(inter)) for conditions of (a) NLSP = 500 cm−3 and (b) NLSP = 2500 cm−3. DNLSP (intra) and DNLSP (inter)are shown as positive values. The other parameters for the initial lognormal size distribution (LNSD) areNLSP = 500, 1500, and 2500 cm−3; NBC = 0.1 × NLSP; CMDLSP = 200, 230, and 250 nm; CMDBC (shell) =120, 160, and 200 nm; sLSP = 1.2 and 1.4; and sBC = 1.2 and 1.3.


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of Figure 20a. This can largely explain the observed increasein Dp/DBC ratio by a factor of about 1.2 in the flaming phaseplumes.[92] BC was thickly coated 1–2 h after emission, as dis-

cussed in section 8.2.1 (Figure 15). Near BB sources,NSC canbe substantially higher than 2500 cm−3. In these areas, thecoating of BC may occur in 1–2 h, considering the resultsshown in Figure 20d. Coagulation can play an important rolein coating BC from early stages after BC emission from BB.[93] Previous studies considered only condensation pro-

cess to explain the evolution of the BC mixing state inplumes strongly impacted by anthropogenic emission [e.g.,Moteki et al., 2007; Shiraiwa et al., 2007, 2008]. For thefirst time, we suggest that coagulation of LSP with BC canalso effectively increase Dp/DBC ratios based on the detailedcomparison of observations and model calculations.

9. Conclusions

[94] We have presented multiple analyses of the ARCTASchemical data combined with meteorological data to quantifythe emissions (including secondary aerosol formation) ofdifferent types of aerosols (black carbon (BC) and organicand inorganic species) from biomass burning (BB) thatoccurred in North America (Canada and California) and

Asia, especially in terms of modified combustion efficiency(MCE). The microphysical properties of aerosols (size dis-tribution and mixing state) emitted from BB were alsoquantified. Data strongly impacted by BB were unambigu-ously extracted using tracers, namely CH3CN and CH2Cl2.We also selected data with low accumulated precipitationalong trajectory (APT) values to minimize the effect of wetremoval of aerosol on the statistical analysis of the aerosolemissions from BB. Below, we summarize the most impor-tant parameters and processes presented above, which can beused directly for improving model estimates of the impact ofBB in the Arctic.[95] In North American plumes, organic aerosol (OA)

constituted about 83% of the total PM1, and the sulfatefraction was only 8% on average. By contrast, in the Asianplumes of the flaming phase, the sulfate fraction was as highas about 27%, with an OA fraction of about 60%, suggestinglarge SO2 emissions from BB in Asia followed by oxidationduring transport. In support of this interpretation, the sulfate +potential sulfate mass fractions were as high as 15%–20% inthe BB plumes of the flaming phase in North America.[96] In all types of BB plumes, the BC mass (volume)

factions of the total PM1(LSP) were 1%–5% (2%–4%), whilethe BC volume fractions were about 30% for the particlescontaining BC, consistent with the fact that most particles

Figure 20. Time series of the ratios of the total shell volume of BC at time t (V (t) =Rp [Dp (t)]

3dDp/6)to the initial V (t = 0) shown together with the initial LNSD parameters. (a) Calculations for the conditionsof CMDBC (shell) = 160 nm. The lines with triangles, squares, and circles are the results for initialCMDLSP = 250, 230, and 200 nm, respectively. (b) Same as Figure 20a but for CMDBC (shell) =120 nm. (c and d) Same as Figure 20a but for initial NLSP of 1500 and 2500 cm−3, respectively.


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did not contain BC. The DMBC/DVSC ratios were muchsmaller than those for BC particles emitted from fossil fuelcombustion.[97] We derived the average emission ratios of MBC/CO =

2.3 ± 2.2 ng m−3/ppbv and MBC/CO2 = 180 ± 269 ng m−3/ppmv for BB in North America. For BB plumes from Asia,the average ratios were MBC/CO = 8.5 ± 5.4 ng m−3/ppbvand MBC/CO2 = 129 ± 67 ng m−3/ppmv. The much higherMBC/CO ratios for the Asian plumes were likely due tohigher MCE. The MBC/CO ratios in North America werelower than the values compiled by Andreae and Merlet[2001] by a factor of 2–4, although the ratios for theAsian plumes were similar. The MBC/CO2 emission ratiosfor North America and Asia were 4–5 times lower than theirestimates.[98] We related the MCE to the emission ratios between

DMBC, DVSC, DCO2, and DCO for BB in North Americaand Asia. The DMBC/DCO2 ratios decreased with theincrease in MCE, whereas the DMBC/DCO ratios increasedwith the increase in MCE. The dependence of the ratios onMCE is useful in improving emissions estimate of aerosolsfrom BB and estimating the transport efficiency of aerosolsby using tracers, especially CO.[99] The shapes of the size distributions of BC were

similar for plumes originating from BB in North Americaand Asia. The distribution functions were represented to agood approximation by lognormal functions. The countmedian diameters (CMDs) of BC were 136–141 nm (±6–8 nm), with standard deviations of 1.32–1.36 (±0.01–0.04)and little dependence on the phase of combustion.[100] These BC particles were thickly coated by organics,

withDp/DBC ratios of about 1.4 (0.6 <MCE < 0.8) 1–2 h afteremission. In the smoldering phase, the Dp/DBC ratiosincreased from 1.35 to 1.45 within about 12 h, with a corre-sponding increase in the volume of nonrefractory species onBC by 15%–40%. In aged flaming phase plumes, theincrease in Dp/DBC ratios continued for a few days, leadingto an increase of the volume of coating materials by a factorof 2. A potential explanation is that OA was being redis-tributed via the gas phase [e.g., Marcolli et al., 2004] byevaporating from particles that did not contain BC andrecondensing on BC‐containing particles.[101] We assessed the effect of the Brownian coagulation

of LSP with BC on the evolution of the BC mixing state. Forthe observed conditions, coagulation can increase Dp/DBC

ratios by factors of 1.2–1.5 in 1–2 days, depending onvarious parameters, influencing the rate of coagulation.Previous studies considered only condensation process toexplain the evolution of the BC mixing state. For the firsttime, we suggest that coagulation of LSP with BC caneffectively increase Dp/DBC ratios, as well as condensation.

[102] Acknowledgments. The ARCTAS mission was supported byNASA. We are indebted to all the ARCTAS participants for their cooper-ation and support. Special thanks are due to the flight and ground crews ofthe NASA DC‐8 aircraft. P. O. Wennberg provided the SO2 data used forthe present study. We thank M. Osuka for his assistance with the field mea-surements. R. Yokelson provided very useful comments on the manuscript.The meteorological data were supplied by the European Centre forMedium‐Range Weather Forecasts (ECWMF). This work was supportedin part by the Ministry of Education, Culture, Sports, Science, and Technol-ogy (MEXT); the Strategic International Cooperative Program of the JapanScience and Technology Agency (JST); and the Global Environment

Research Fund of the Japanese Ministry of the Environment (B‐083). Y.Z.was supported in part by the NASA Tropospheric Chemistry Program(USP‐SMD‐08‐009). M.J.C. and J.L.J. acknowledge NASA grantNNX08D39G. CH3CN measurements were supported by the AustrianResearch Promotion Agency (FFG‐ALR) and the Tiroler Zukunftstiftung,operated with the help and support of M. Graus, A. Hansel, and T. D. Maerk.

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Physics, University of Innsbruck, Technikerstr. 25, A‐6020 Innsbruck,Austria. ([email protected]; [email protected])L. Sahu, Physical Research Laboratory, Ahmedabad 380009, India.

([email protected])Y. Zhao, Air Quality Research Center, University of California, One

Shields Ave., Davis, CA 95616, USA. ([email protected])


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